U.S. Pat. No. 4,062,314 discloses a magnetically operated door chime. In this patent a striker pin is mounted in a housing for vertical reciprocation adjacent the upper edge of a door, and a magnetically polarized piece is provided at the upper end of the striker pin. A chime bar is mounted on the housing below the lower end of the striker pin and a spring yieldably biases the striker pin upwardly out of contact with the chime bar. A magnet is mounted by a holder on the door frame at a location to overlie the magnetically polarized piece on the striker pin when the door is closed, and the magnet is arranged to magnetically attract and raise the striker pin when the door is closed. When the door is opened, the magnetically polarized piece on the striker pin moves out of a position below the magnet on the door frame and allows the striker pin to fall with the aid of gravity and strike the chime bar. A second magnet polarized opposite to the polarity of the magnetically polarized piece on the striker pin is provided on the holder attached to the door frame at a location such that the magnetically polarized piece on the striker pin moves under the second magnet as the door is opened and the second magnet repels the striker pin and aids in moving the same downwardly toward the chime bar.
The magnetic attracting and repelling forces between two magnets decreases rapidly as the air gap therebetween increases. In the magnetically operated door chime disclosed in U.S. Pat. No. 4,062,314, a substantial minimum air gap must be provided between the magnet on the striker carried by the door and the magnet on the holder carried by the door frame to allow clearance for movement of the door between open and closed positions. In addition, the magnets must operate through an air gap that is substantially greater than the minimum air gap in order to attract and raise the striker pin preparatory to a chime operating stroke of the pin. Changes in ambient temperature and/or humidity can change the operating clearance between the door and door frame and affect the minimum air gap. In addition, the need for the magnet to operate through a relatively long air gap in order to raise the striker, limits the maximum possible stroke of the striker and hence the impact forces available to move the striker against the chime bar.